It has been previously demonstrated that the addition of alkyl aluminum species, including aluminoxanes, to a high pressure, free radical polyethylene reaction can be beneficial in terms of molecular catalyst activation, and as a scavenger for polar species. It has also been shown that alkyl aluminum species, in a high pressure polyethylene reactor, cause catalytic chain transfer, reducing the polyethylene molecular weight (raising the melt index). This can be advantageous for the production of certain products, but can also be disadvantageous in certain applications, such as films, since the production of low molecular weight (MW) polymer may impair the film properties. More recently, it has been discovered that, at elevated temperatures and concentrations, these alkyl aluminum species can break apart to form free radicals, leading to runaway reactions, again an undesirable effect. Alkyl aluminums have been used in the high pressure polyethylene reactors at low temperatures or concentrations to avoid the instability or chain transfer effects.
Thus, there is a need for new free-radical polymerizations for the production of ethylene-based polymers that reduce, or eliminate, both the chain transfer effect and the instability of the alkyl aluminum at high temperatures, without hindering the ability of the alkyl aluminum to scavenge polar impurities or activate molecular catalysts.
J. Schellenberg, J. Macromol. Rapid Commun. 2005, 26, 1299-1303, discloses a solvent-free, syndiospecific, coordination polymerization of styrene, in the presence of MAO, TIBA, and the reaction products of sterically hindered phenolic compounds. The presence of such reaction products was disclosed as increasing the polymerization activity of the catalyst system, and improving the thermal stability of the polymers produced.
Turunen et al., Journal of Applied Polymer Science, Vol. 100, 4632-4635 (2006), discloses the use of a sterically hindered phenol to reduce the catalyst leaching of zirconocene dichloride (Cp2ZrCl2) from a silica support during an ethylene polymerization. Busico et al., J. Am. Chem. Soc. 2003, 125, 12402-12403 discloses the controlled addition of a sterically hindered phenol to MAO solutions, as a way to trap “free” trimethylaluminum (TMA).
Carlini et Al., Macromol. Rapid Commun. 2005, 26, 808-812, discloses the use of 2,6-di-tert-butylphenol in an ethylene polymerization, catalyzed by a bis(salicylaldiminate)-nickel(II)-based catalyst, to reduce the content of free trimethylaluminum. European Patent Application EP1650236A1 discloses a metallocene catalyst system, prepared with a hafnium-based metallocene catalyst and an activating agent comprising an aluminoxane and a sterically hindered Lewis base.
Additional polymerization processes and/or polymer products are disclosed in the following references: Huff et al., Reaction of Polymeric Radicals with Organoaluminum Compounds, Journal of Polymer Science: Part A, 1963, 1, 1553-1572; Huff et al., The Reaction of Styryl Radicals with Organoaluminum Compounds, J. Am. Chem. Soc., 1960, 82, 4277-4281; Grotewold et al., Triethylaluminum as a Concentrate-Dependent Coinitiator and Chain-Transfer Agent of Free-Radical Polymerization of Methyl Methacrylate in the Presence of Benzoquinone, Journal of Polymer Science: Polymer Chemistry Edition, 1977, 15, 393-404; Milovskaya et al., Synthesis and Characteristics of Polystyryl Aluminum Derivatives and Their Reaction with Benzoyl Peroxide, Polymer, 1982, 23, 891-896; Götz et al., Influence of Aluminum Alkyl Compounds on the High-Pressure Polymerization of Ethylene with Ternary Metallocene-Based Catalysts. Investigation of Chain Transfer to Aluminum, Macromol. Mater. Eng., 2002, 287, 16-22; Gridnev et al., Catalytic Chain Transfer in Free-Radical Polymerizations, Chem. Rev., 2001, 101, 3611-3659; Mortimer, Chain Transfer in Ethylene Polymerization. VII. Very Reactive and Depletable Transfer Agents, Journal of Polymer Science: Part A-1, 1972, 10, 163-168; International Publication Nos. WO 2010/042390, 2011/019563, 1997/45465; European Application Nos. EP 2256158A1, EP2256159A1; U.S. application Ser. No. 12/701,859; U.S. Pat. Nos. 7,767,613; 6,521,734; 5,539,075; U.S. Publication 2010/0108357;
However, the polymerizations discussed above are directed to coordination polymerizations, catalyzed by metallocene and other coordination-type catalysts, and typically run at low pressures and low temperatures. These low pressure polymerization processes do not have the same issues of catalytic chain transfer and free radical runaway reactions associated with free-radical polymerizations. As discussed above, there is a need for new free-radical polymerizations to form ethylene-based polymers, such as low density polyethylene (LDPE), that can be run in the presence of an alkyl aluminum compound, for the activating and scavenging benefits of the alkyl aluminum compound, and which also reduce, or eliminate, the chain-transfer effects and instability of the alkyl aluminum compound, especially at higher polymerization temperatures. These needs have been met by the following invention.